The present invention pertains to an oxygen fueled combustion system. More particularly, the present invention pertains to an oxy-fueled combustion system in which the production of green-house gases is reduced and in which fossil fuel consumption is reduced.
Oxygen fueled burner systems are known, however, their use is quite limited. Oxy-fueled burner systems are generally only used in those applications in which extremely high flame temperatures are required. For example, these systems may be used in the glass making industry in order to achieve the temperatures necessary to melt silica to a fusion temperature. Otherwise, it is commonly accepted that structural and material limitations dictate the upper temperatures to which many industrial systems can be subjected. To this end, air fueled or air fired combustion systems are used in boilers, furnaces and the like throughout most every industrial application including manufacturing, electric power generating and other process applications.
In particular, air fueled combustion systems or electric heating systems are used throughout the steel and aluminum making industries, as well as the power generation industry, and other industries that rely upon carbon based fuels. In air fueled systems, air which is comprised of about 79% nitrogen and 21% oxygen, is fed, along with fuel into a furnace. The air fuel mixture is ignited creating a continuous flame. The flame transfers energy in the form of heat from the fuel air mixture into the furnace.
In the steel and aluminum industries, air fueled furnaces and electric furnaces have been used as the primary heat source for creating molten metals. With respect to air fueled furnaces, it is conventionally accepted that the energy requirements, balanced against the thermal limitations of the process equipment, mandate or strongly support the use of these types of combustion systems. As to the use of electric furnaces in the aluminum industry, again, conventional wisdom supports this type of energy source to achieve the temperatures necessary for aluminum processing.
One drawback to the use of air fueled combustion systems, is that these systems produce NOx and other green-house gases such as carbon dioxide, sulfur dioxide and the like, as an inherent result of the combustion process. NOx and other green-house gases are a large contributor to environmental pollution, including, but not limited to acid rain. As such, the reduction in emission of NOx and other green-house gases is desirable, and as a result of regulatory restrictions, emission is greatly limited. To this end, various devices must be installed on these combustion systems in order to limit and/or reduce the levels of NOx and other green-house gases produced.
Another drawback with respect to air fueled furnaces is that much of the energy released from the combustion process is absorbed or used to heat the gaseous nitrogen present in the air that is fed to the furnace. This energy is essentially wasted in that the heated nitrogen gas is typically, merely exhausted from the heat source, e.g., furnace. To this end, much of the energy costs are directed into the environment, through an off-gas stack or the like. Other drawbacks of the air fed combustion systems known will be recognized by skilled artisans.
Electric furnaces likewise have their drawbacks. For example, inherent in these systems as well is the need for a source of electricity that is available on a continuous basis, essentially without interruption. In that large amounts of electric power are required to operate electric furnaces, it is typically necessary to have these electric furnaces located in proximity to electric generating plants and/or large electrical transmission services. In addition, electric furnaces require a considerable amount of maintenance to assure that the furnaces are operated at or near optimum efficiency. Moreover, inherent in the use of electric furnaces is the inefficiency of converting a fuel into electrical power (most large fossil fueled power stations that use steam turbines operate at efficiencies of less than about 40 percent, and generally less than about 30 percent). In addition, these large fossil fueled stations produce extremely large quantities of NOx and other green-house gases.
For example, in the aluminum processing industry, and more specifically in the aluminum scrap recovery industry, conventional wisdom is that flame temperatures in furnaces should be maintained between about 2500xc2x0 F. and 3000xc2x0 F. This range is thought to achieve a balance between the energy necessary for providing sufficient heat for melting the scrap aluminum, and maintaining adequate metal temperatures in the molten bath at about 1450xc2x0 F. Known furnaces utilize a design in which flame temperatures typically do not exceed 3000xc2x0 F. to assure maintaining the structural integrity of these furnaces. That is, it is thought that exceeding these temperature limits can weaken the support structure of the furnace thus, possibly resulting in catastrophic accidents. In addition, stack temperatures for conventional furnaces are generally about 1600xc2x0 F. Thus, the temperature differential between the flame and the exhaust is only about 1400xc2x0 F. This results in inefficient energy usage for the combustion process.
It is also believed that heat losses and potential damage to equipment from furnaces in which flame temperatures exceed about 3000xc2x0 F. far outweigh any operating efficiency that may be achieved by higher flame temperatures. Thus, again conventional wisdom fully supports the use of air fueled furnaces in which flame temperatures are at an upper limit of about 3000xc2x0 F. (by flame stoichiometry) which assures furnace integrity and reduces energy losses.
Accordingly, there exists a need for a combustion system that provides the advantages of reducing environmental pollution (attributable to NOx and other green-house gases) while at the same time providing efficient energy use. Desirably, such a combustion system can be used in a wide variety of industrial applications, ranging from the power generating/utility industry to chemical processing industries, metal production and processing and the like. Such a combustion system can be used in metal, e.g., aluminum, processing applications in which the combustion system provides increased energy efficiency and pollution reduction. There also exists a need, specifically in the scrap aluminum processing industry for process equipment (specifically furnaces) that are designed and configured to withstand elevated flame temperatures associated with such an efficient combustion system and to increase energy efficiency and reduce pollution production.
An oxygen fueled combustion system includes a furnace having a controlled environment, and includes at least one burner. The combustion system includes an oxygen supply for supplying oxygen having a predetermined purity and a carbon based fuel supply for supplying a carbon based fuel. The present oxy fuel combustion system increases the efficiency of fuel consumed (i.e., requires less fuel), produces zero NOx (other than from fuel-borne sources) and significantly less other green-house gases.
The oxygen and the carbon based fuel are fed into the furnace in a stoichiometric proportion to one another to limit an excess of either the oxygen or the carbon based fuel to less than 5 percent over the stoichiometric proportion. The combustion of the carbon based fuel provides a flame temperature in excess of about 4500xc2x0 F., and an exhaust gas stream from the furnace having a temperature of not more than about 1100xc2x0 F.
The combustion system preferably includes a control system for controlling the supply of carbon based fuel and for controlling the supply of oxygen to the furnace. In the control system, the supply of fuel follows the supply of oxygen to the furnace. The supply of oxygen and fuel is controlled by the predetermined molten aluminum temperature. In this arrangement, a sensor senses the temperature of the molten aluminum.
The carbon based fuel can be any type of fuel. In one embodiment, the fuel is a gas, such as natural gas, methane and the like. Alternately, the fuel is a solid fuel, such as coal or coal dust. Alternately still, the fuel is a liquid fuel, such a fuel oil, including waste oils.
In one exemplary use, the combustion system is used in a scrap aluminum recovery system for recovering aluminum from scrap. Such a system includes a furnace for containing molten aluminum at a predetermined temperature, that has at least one burner. The recovery system includes an oxygen supply for supplying oxygen to the furnace through the combustion system. To achieve maximum efficiency, the oxygen supply has an oxygen purity of at least about 85 percent.
A carbon based fuel supply supplies a carbon based fuel. The oxygen and the carbon based fuel are fed into the furnace in a stoichiometric proportion to one another to limit an excess of either the oxygen or the carbon based fuel to less than 5 percent over the stoichiometric proportion. The combustion of the carbon based fuel provides a flame temperature in excess of about 4500xc2x0 F., and an exhaust gas stream from the furnace having a temperature of not more than about 1100xc2x0 F.
In such a recovery system, the combustion of oxygen and fuel creates energy that is used for recovering aluminum from the scrap at a rate of about 1083 BTU per pound of aluminum recovered. The fuel can be a gas, such as natural gas, or it can be a solid fuel or a liquid fuel.
In the recovery system, heat from the furnace can be recovered in a waste heat recovery system. The recovered heat can be converted to electrical energy.
In a most preferred system, the combustion system includes a system for providing oxygen. One such system separates air into oxygen and nitrogen, such as a cryogenic separation system. Other systems include membrane separation and the like. Oxygen can also be provided by the separation of water into oxygen and hydrogen. In such systems, the oxygen can be stored for use as needed. Other systems are known for oxygen generation/separation.
The oxygen fueled combustion system, generally, can be used with any furnace that has a controlled environment. That is, with any furnace that has substantially no in-leakage from an external environment. Such a combustion system includes an oxygen supply for supplying oxygen having a predetermined purity and a carbon based fuel supply for supplying a carbon based fuel.
The oxygen in the oxygen supply and the carbon based fuel are fed into the furnace in a stoichiometric proportion to one another to limit an excess of either the oxygen or the carbon based fuel to less than 5 percent over the stoichiometric proportion. In such a furnace, an exhaust gas stream from the furnace has substantially zero nitrogen-containing combustion produced gaseous compounds. That is, because there is no nitrogen fed in with the fuel, unless there is fuel-borne nitrogen, the exhaust gas contains substantially no nitrogen containing combustion products (i.e., NOx), and significantly lowered levels of other green-house gases.
This combustion system can use any carbon based fuel including gas, such as natural gas or methane, any solid fuel such as coal or coal dust or any liquid fuel, such as oil, including waste and refined oils. In such a combustion system, any nitrogen-containing combustion produced gaseous compounds are formed from the fuel-borne nitrogen.
A method for recovering aluminum from scrap includes feeding aluminum scrap into a melting furnace and combusting oxygen and a carbon based fuel in the furnace. In the combustion of the oxygen and fuel, the oxygen and fuel are fed into the furnace in a stoichiometric proportion to one another to limit an excess of either the oxygen or the carbon based fuel to less than 5 percent over the stoichiometric proportion. The combustion provides a flame temperature in excess of about 4500xc2x0 F., and an exhaust gas stream from the furnace having a temperature of not more than about 1100xc2x0 F.
The aluminum is melted in the furnace, contaminant laden aluminum is removed from the furnace and substantially pure molten aluminum is discharged from the furnace. The method can include the step of recovering aluminum from the contaminant laden aluminum, i.e., dross, and charging the recovered aluminum into the furnace.
The method can include recovering waste heat from the furnace. The waste heat recovered can be converted to electricity.
A furnace for recovering aluminum from scrap aluminum includes a bath region for containing molten aluminum at a predetermined temperature, and at least one burner. An oxygen supply supplies oxygen having a purity of at least about 85 percent and a carbon based fuel supply supplies fuel, such as natural gas, coal, oil and the like.
The oxygen in the oxygen supply and the fuel are fed into the furnace in a stoichiometric proportion to one another to limit an excess of either the oxygen or the fuel to less than 5 percent over the stoichiometric proportion. The combustion of the fuel provides a flame temperature in excess of about 4500xc2x0 F., and an exhaust gas stream from the furnace has a temperature of not more than about 1100xc2x0 F.
In one embodiment, the furnace is formed from steel plate, steel beams and refractory materials. The furnace walls are configured having a steel beam and plate shell, at least one layer of a crushable insulating material, at least one layer of a refractory brick, and at least one layer of a castable refractory material. The furnace floor is configured having a steel beam and plate shell and at least two layers of refractory material, at least one of the layers being a castable refractory material.
A salt-less method for separating aluminum from dross-laden aluminum is also disclosed that includes the steps of introducing the dross-laden aluminum into a furnace. The furnace has an oxygen fuel combustion system producing a flame temperature of about 5000xc2x0 F., and having substantially no excess oxygen. The dross-laden aluminum melts within the furnace.
An upper portion of the melted dross-laden aluminum is skimmed to produce a heavily dross-laden product. The heavily dross-laden product is pressed in a mechanical press to separate the aluminum from the heavily dross-laden product to produce a concentrated heavily dross-laden product. The method can include the step of returning the concentrated heavily dross-laden product to the furnace. Introduction of the dross-laden aluminum into the furnace is carried out in near direct flame impingement to release the oxides from the dross.
These and other features and advantages of the present invention will be apparent from the following detailed description, in conjunction with the appended claims.